Fiber-reinforced carbon-containing materials of this type suffer from the problem that the carbon tends to undergo oxidation, in particular in relatively high-temperature applications. This behavior is particularly disadvantageous when shaped bodies used in a high-temperature environment, for example furnace linings, combustion chambers or brake discs of vehicles, are to be made of the carbon-containing material. Brake discs are, for example, manufactured from composites reinforced with carbon fibers and having a ceramic matrix such as C/SiC materials (silicon carbide containing Si and/or C phases and reinforced with carbon fibers). In the case of such brake discs, undesirable burning has in the past been observed at operating temperatures of from abut 600° C. to above 1000° C.
Starting points for protecting these materials against oxidation are therefore known from the prior art. For example, EP 0 619 801 B1 describes a process of this generic type for protecting fiber-reinforced, carbon-containing composites whose matrix comprises silicon carbide (SiC) at least in the outer layer against oxidation. More precisely, the starting material is formed by a C-SIC material which has a pure SiC matrix which is deposited from the gas phase and in which no free Si is present.
The known process comprises, inter alia, the following steps:                a) forming an outer coating on the ceramic surface of the composite by means of a composition comprising a mixture of phosphates and silicon oxide and aluminum oxide,        b) after drying, carrying out a heat treatment at a temperature which is sufficient to convert the outer coating into an insoluble cement which is suitable for forming a self-healing glass.        
The outer coating is produced by brushing or spraying the aqueous, phosphate-containing suspension onto the ceramic surface of the composite. The conversion of the coating which forms a cement after heat treatment into a self-healing glass then occurs during the use of the material at an operating temperature which is greater than or equal to the softening temperature or is carried out by means of a further heat treatment above the softening temperature. For the purposes of the present invention, a self-healing glass is a glass which automatically closes surface cracks and damage occurring over time at elevated temperatures and without external intervention.
It is an object of the invention to develop a process of the type mentioned at the outset further so as to achieve improved oxidation protection of the composite material. The production of the material should be carried out in such a way that the process is not significantly more costly or time-consuming compared to methods of production used hitherto.
In contrast to the prior art, in which a fiber-reinforced, carbon-containing composite whose matrix comprises silicon carbide (SiC) but no free silicon (Si) is protected against oxidation, according to the invention it is a composite comprising, at least in the outer layer, not only silicon carbide (SiC) but also silicon (Si) and/or silicon alloys which is protected against oxidation. In conjunction with the additional oxidative treatment, silicon oxide (SiO2) is produced from the silicon (Si) or its alloys present in the matrix. The layer formed in the phosphating step reacts with the free silicon (Si) present in the matrix or a silicon oxide formed therefrom as an intermediate to form a silicate glass having self-healing properties, preferably Al2O3—SiO2—P2O5. The presence of silicon in the matrix of the composite therefore has the advantage that formation of the self-healing glass phase occurs during production of the composite. This plays a role, in particular, in the case of vehicle brake discs which are produced from the composite and are subjected to oxidative stresses even before a first use at elevated temperatures, for example when new vehicles from the factory are kept in the open before dispatch to the point of sale. Furthermore, the presence of the silicon bound in the matrix of the initial material accelerates and aids the production of the self-healing glass which is advantageous for affording oxidation protection. This is particularly important when the finished product is used under conditions under which further amounts of the glass-forming SiO2 are continually formed from the matrix and can be incorporated into the self-healing glass. Finally, this results in better bonding of the glass phase to the composite because the silicon is already present in the matrix of the latter and is not applied only to the surface as in the prior art.